Technical Field of the Invention
The present invention relates to plastic containers having enhanced barrier properties and, in particular, plastic containers having enhanced barrier properties retained after thermal processing such as, for example, in applications where the container and its contents are heated up to 270° F. Such applications include sterilization, pasteurization or retort processes. The present invention also relates to a process of manufacturing such plastic containers.
Background Art
Blow molding processes for forming PET containers are well known in the art. PET plastic containers have replaced or provided an alternative to glass containers for many applications. Some food products that are packaged in blow molded PET containers must at least filled at a high temperature, i.e., up to 195° F. and, in some instances, must be processed using pasteurization or retort at a temperature of up to 230° F. and above. Pasteurization and retort methods are frequently used for sterilizing solid or semi-solid food products, e.g., pickles and sauerkraut. The products may be packed into the container along with a liquid at a temperature less than 180° F. and then sealed and capped, or the product may be placed in the container that is then filled with liquid, which may have been previously heated, and the entire contents of the sealed and capped container are subsequently heated to a higher temperature. As used herein, “high-temperature” pasteurization and retort are sterilization processes in which the product is exposed to temperatures of from about 176° F. to about 270° F.
Pasteurization and retort differ from hot-fill processing by including heating the filled container to a specified temperature, typically greater than 200° F., until the contents of the filled container reach a specified temperature, for example 175° F., for a predetermined length of time. That is, the external temperature of the hot-filled container may be greater than 200° F. so that the internal temperature of a solid or semi-solid product reaches approximately 175° F. Pasteurization and retort processes may also involve applying overpressure to the container. The rigors of such processing present significant challenges for the use of plastic containers having high gas barrier requirements to fluids such as, for example, oxygen (ingress) and carbon dioxide (egress). One excellent barrier material is a silicon oxide based barrier coating that is applied typically to the inside of the container using commercially available vacuum chemical vapor deposition methods. The Barrier Improvement Factor (BIF) for these freshly coated containers can be as high as 200×, or higher, compared to the same container without the silicon oxide coating. During pasteurization and retorting processes, the container is typically distorted by the high temperatures and internal pressure in the container that can stress the silicon oxide coating and form micro fractures in the coating. This results in a deterioration of the BIF by 20× or greater, thus yielding a net result BIF of not greater than 10×. Ultimately, of course, the deteriorated BIF translates into a shortened shelf life for the packaged product.
Accordingly, there is a need in the art for a process to produce a blow molded PET container that has a silicon oxide based barrier coating that can withstand the rigors of a pasteurization or retort process without sacrificing a significant portion of the BIF gained by application of the coating.